1. Field of the Invention
This invention relates to a method of dissolving and removing organic filter cakes from the face of a formation fracture and formation pores utilizing a controlled pH fluid.
2. Description of the Prior Art
Hydraulic fracturing is commonly employed to increase the production of fluids from a subterranean formation. Hydraulic fracturing involves the injection of a suitable fracturing fluid down a well penetrating a formation and into the formation under sufficient pressure to create a crack or fracture in the formation. The created crack or passageway facilitates the flow of fluids through the formation and into the well.
In a typical fracturing treatment a particulate propping agent such as sand, sintered bauxite, glass beads, alumina, resin coated sand or the like is introduced into the created fracture to assist in keeping the fracture at least partially propped in an open position. The propped open passageway further enhances the flow of formation fluids. To facilitate transport of the proppant material into the created fractures, the fracturing fluid typically is viscosified with one or more of the various well know polysaccharide or synthetic polymer gelling agents. The gelling agent imparts viscosity to the fluid thereby permitting enhanced proppant transport and reduces fluid loss from the fracturing fluid to the formation. As the gelled fracturing fluid is introduced into contact with the subterranean formation, a portion of the fluid leaks off into the formation. As the fluid leaks off into the formation matrix, the gelling agent is stripped from the fluid. The stripped gelling agent is deposited on the face of the created fracture and is concentrated thereon by the action of filtration as additional fluid leaks-off. This layer of gelling agent often is referred to as "filter cake" and is desirable during fracture initiation and propagation because it minimizes fluid loss from the fracturing fluid. The gelling agent often is admixed with a crosslinking agent to further enhance the viscosity of the fracturing fluid. While the crosslinked gels are particularly effective at reducing fluid loss and effecting proppant placement, they form a particularly tough, leathery filter cake which is generally insoluble in aqueous fluids.
Combination fracture-acidizing processes also are well know in the art. In such processes an aqueous acid is utilized in the formation resulting in enhanced formation permeability.
Upon conclusion of the fracturing treatment the fracture is permitted to close upon the propping agent distributed within the fracture. The closure may result in a portion of the filter cake being forced into the proppant bed within the fracture thereby plugging the flow channels in close proximity to the face of the fracture. These flow channels often exhibit the highest fluid conductivity. The presence of the filter cake significantly reduces the fracture flow capacity. Also, a portion of the filter cake may be forced into the formation pores reducing the matrix permeability.
It would be desirable to provide a method by which the filter cake can be removed from the face of the fracture in the formation and the pores of the formation matrix and thereby increase the fracture flow capacity.